This study had four aims:

1. To evaluate the correlation between the planned preoperative cup size and the actual cup size that was implanted in surgery.
2. To examine the absolute difference between the implanted cup size diameter and the actual femoral head size diameter.
3. To examine the absolute difference between the planned cup size diameter and the actual femoral head size diameter
4. To suggest an additional online monitoring tool on the accuracy of the preoperative process and implant selection. We hypothesized that high accuracy rates will be found between the preoperative planned cup size and the actual implanted cup size. Furthermore, we hypothesized that strong correlations will be found between the planned and actual implanted cup size and the actual femoral head diameter size.

Methods This is a retrospective study , that will include patients who underwent primary cementless THA due to end-stage hip osteoarthritis during 2011 and 2013 . Inclusion criteria were: Patients that underwent primary THA due to end-stage primary osteoarthritis; Males and females between 15-90 years of age.

Data collection The following data will be collected: patients' characteristic including gender, age at surgery and side of surgery; implant information including cup model, planed cup size, actual cup size, shell type and outer diameter size (mm); actual femoral head diameter size (mm); the difference between the planned cup size and the actual cup size that was used; the difference between the actual femoral head diameter size measured at the end of surgery and the cementless implanted cup outer diameter size; the difference between the actual femoral head diameter size and the planed cup size..

Inclusion Criteria:

• Patients that underwent primary Total hip arthroplasty due to end-stage primary osteoarthritis;

Exclusion Criteria:

• Significant distortion in the femoral head.
• Previous reconstructive surgery - radial osteotomy and hip socket.

Patients that underwent primary Total hip arthroplasty due to end-stage primary osteoarthritis; Males and females between 15-90 years of age.